In conventional digital communication systems, a sequence of reference symbols called reference symbols or pilot symbols, known to the receiver, is frequently used in the data stream sent by the transmitter. These reference symbols enable a receiver to suitably estimate the transmission channel and thus guarantee that received signals are suitably demodulated.
For communication systems involving transmission channels variable with time, it has been envisaged to distribute reference symbols in different positions within an exchanged data stream.
In these various communication systems, the structure and characteristics of reference symbols of a radio communication device are determined as a function of the worst propagation case. This type of constraint is essential in order to make a correct channel estimate, regardless of propagation conditions.
When the initial synchronisation phase is complete, a dedicated resource will be allocated to the radio communication terminal, and it will then implement a synchronisation holding phase if necessary, and make an estimate of the transmission channel.
According to this prior art technique, the reference structure is chosen so as to enable a correct channel estimate in all cases, and even in the worst communication case, in other words for a maximum multipath level and the maximum speed of the communication terminal considered.
In other words, the reference structure is sized to be adapted to the worst Doppler case and the worst delay spread.
A disadvantage of sizing the reference structure for the worst case is that it induces a statistical loss of useful transmission capacity and/or protection against errors, in cases in which the characteristics of the transmission channel are good.
The inventors have detected and analysed this problem. They have deduced that a disadvantage of this technique according to prior art is that the reference structure is frozen once and for all, for a given physical transmission channel.
Consequently, another disadvantage of this technique according to prior art is that the selected sequence of reference symbols does not depend on the characteristics of the transmission channel that depend partly on the environment of the communication terminal considered, and also on the displacement speed of this terminal.
Note that simply stating this problem is new and inventive to one skilled in the art, since the latest has always considered that the reference structure should be fixed and unique in order to produce a good channel estimate, since it is adapted to the worst signal propagation case.